Low power self-limiting input circuit

Abstract

An input circuit for an input module having a plurality of inputs is disclosed. Each input circuit has a reduced number of components, providing for an increased number of inputs within an input module of comparable size to prior art input modules. Alternately, the input module may have a like number of inputs in a reduced size in comparison to prior art input modules. The reduced number of components similarly reduces the power dissipated by each input circuit.

We claim: 
     
         1 . An input circuit for an industrial controller, comprising:
 a first terminal and a second terminal configured to receive a voltage potential, wherein the first terminal is configured to be a positive terminal for the voltage potential with respect to the second terminal;   a zener diode having a cathode and an anode, the cathode electrically connected to the first terminal;   a current limit diode having a cathode and an anode, the anode of the current limit diode electrically connected to the anode of the zener diode; and   an opto-coupler having a light emitter and a photosensor, the light emitter electrically connected in series between the cathode of the current limit diode and the second terminal.   
     
     
         2 . An input circuit for an input module in an industrial controller, comprising:
 an input configured to receive a range of direct current (DC) voltages;   a zener diode electrically connected to the input to operate in a breakdown region when the DC voltage is applied to the input;   a current limit diode electrically connected in series with the zener diode to operate in a current limiting mode when the DC voltage is applied to the input; and   an opto-coupler having an input and an optically isolated output wherein the input is electrically connected to the series-connected zener diode and current limit diode and the optically isolated output generates a signal that corresponds to a logical true when the DC voltage is applied to the input and a logical false when the DC voltage is removed from the input.   
     
     
         3 . The input circuit of  claim 2  wherein the range of DC voltages is between 10 volts and 32 volts. 
     
     
         4 . The input circuit of  claim 3  wherein an amplitude of current at the input is greater than or equal to 2 milliamps. 
     
     
         5 . The input circuit of  claim 2  wherein the input includes a first terminal and a second terminal. 
     
     
         6 . The input circuit of  claim 5  wherein the first terminal is configured to receive a voltage potential within the range of DC voltages and the second terminal is configured to be connected to one of a common and a ground. 
     
     
         7 . The input circuit of  claim 6  wherein the second terminal is a shared connection between the input circuit and at least one other input circuit on the input module. 
     
     
         8 . The input circuit of  claim 6  wherein each of the first and second terminals are configured to receive an electrical conductor external to the input module. 
     
     
         9 . The input circuit of  claim 6  wherein the first terminal is configured to receive an electrical conductor external to the input module and the second terminal is configured to be connected internal to the input module.